Treadles placed in or on a road surface have long been employed to monitor traffic in data systems used to provide statistical data upon which highway revisions and designs of future highways are based. In such surveys, the type and size of the vehicle passing a particular location is important. It is important to know whether automobiles or trucks are passing the traffic monitor, and if a truck, it is important that an indication of the number of axles and wheels should be obtained.
Another application of wheel sensing treadles is at toll booths where fees are collected for bridges, tunnels, toll-highways, parking lots, and the like. In such facilities, the toll which is to be collected generally is dependent upon the type of vehicle, and in the case of vehicles that are not automobiles, the toll depends upon the number of wheels and axles of the vehicle. It is important in a vehicle classification system that a treadle measuring system be able to distinguish more than the total number of vehicle axles passing a given location, that is, a revenue collection system should permit identification of vehicle type. In the case of large vehicles, for example, trucks and buses, it is important to know the number of axles and the number of wheels on each axle. It is important that single tire axles each having two wheels be distinguishable from dual tire axles each having four wheels. The tire width is also important in determining what type of vehicle has moved over the treadle.
Many treadle constructions and associated logical systems have been developed. For example, U.S. Pat. No. 3,748,443 to Kroll, et al. and U.S. Pat. No. 3,835,449 to Viracola disclose logical and physical constructions for detecting total tire axle count at a given roadway location, and for providing a breakdown that determines a dual tire axle count and a single tire axle count.
A wheel sensing treadle is generally an elongated rectangular device that in some uses lies entirely across the traffic lane of a highway, and in other applications extends across half or, generally speaking, only a portion of a highway lane. The top surface of the wheel sensing treadle is substantially flush with the roadway surface, and as a vehicle wheel rolls over the treadle, the top treadle surface is locally compressed.
This action of a wheel crossing the treadle initiates electrical signals, generally, through the closing of a switch or switches, although other techniques such as blockage of light in optical fibers has also been used to indicate that a wheel has rolled over the treadle. As described in the prior art, it is possible, by the arrangement of sensors in the treadle, to determine the travel direction of the vehicle moving across the treadle and the number of wheel axles that pass over the treadle. Theoretically, there is an ability to distinguish between a single tire axle with one tire at each axle end, and a dual tire axle with two tires at each axle end.
However, from a practical point of view, it must be noted that heretofore no actual systems having these capabilities are in use. For various reasons, known in the art, it has not been possible to produce a reliable construction that gives the widths of the individual tires and distinguishes single tire axles from dual tire axles. A significant factor in this failure to provide such a system, with reliability, is the spacing and layout of pressure switches in a treadle, as in the patents mentioned above.
In prior art treadles used in wheel sensing, a row of individual switches is generally positioned transversely to the direction of traffic flow. The switches comprise individual metal contacts embedded in a resilient material with an open gap between the contacts. When a tire rolls over the treadle, the treadle is locally compressed; the gap closes and the contacts touch to close the switch. Leads to the individual switch contacts bring signals indicating a closed switch to logical circuits that process the data into a required format.
However, the results from prior art treadles have not been accurate because an insufficiency in the number of switches arranged along the length of the treadle makes for difficult interpretation of the data, and an inability to reliably distinguish different tire combinations and tire widths. Existing treadles provide only a crude measurement of tire width. They cannot distinguish a wide single tire from a narrow dual tire.
The constructions of the prior art switches, wherein upper and lower switch contacts are individual elements, limit the number of switches which can be practically manufactured into a treadle of required length. Further, the difficulty of bringing electrical leads out of the treadle, which is generally long and of low height or thickness, provides a lead density problem that grows as the number of switches increases.
Thus, the vehicle wheel sensing treadles now in actual use have a limited quantity of switches and do no more than count total axles. Measurements of tire widths, and dual tire axles and their tire widths and spacings, have not been accomplished in a practical manner that is suitable for everyday usage in systems requiring high reliability, and quick and easy repair when failures do occur. Existing designs are not reparable.
What is needed is a highly reliable treadle for wheel sensing that can distinguish tire widths, dual and single tire axles, tire spacings and total axle count in an effective and economical manner.